fix: handle zizmor not found case gracefullier

[lishaduck]

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  Some autocomplete jumped up and I might have accepted one or two before getting annoyed as usual. I don't really remember, it's getting late lol. I changed my approach tho, so probably no AI slop in the final PR.

  I had it review my pared down which impl to double check I didn't miss anything, it pointed out some mistakes.

Summary

On macOS, apps don't pick up the path, so you'd have to manually set the zizmor (see #27). This adds in support for falling back to commonly installed locations. This also handles the case that exePath is an empty string, as the setting GUI is prone to do (as I discovered in testing).

Fixes #29

The implementation is shameless copied heavily based & modified from JJK's implementation [MIT], and also includes portions of which [ISC] & (trivially) @types/which [MIT].

Test Plan

Test with it in path, not in path, etc etc.
Seemed to work for me but should check on Windows, etc.